One undisputed consequence of our changing climate is the spread of infectious diseases into new regions as their habitats change. Many so-called tropical diseases are now moving into more temperate latitudes.
While Washington fiddles and California burns, deadly microbes are on the march.
Diseases, particularly those that are carried by mosquitoes, fleas and ticks, are expanding into new regions. No matter where you live in the US, one or more of the infectious diseases they bear is or will soon be in your neighborhood. Recently, The Center for Biological Diversity published an interactive map that shows the tick- and insect-borne diseases which are predicted to enter your locale.
How Skin Is Impacted By The Shifting Landscape Of Infectious Diseases
Global warming is shifting the landscape of infectious diseases, and skin is a player on this field because – at the very least – it provides the ‘portal of entry’ through which microbes can be introduced by these biting ‘arthropods’, (the invertebrate phylum that includes insects and ticks). But beyond serving as the entry point, skin often is involved at an early stage in these infections and its involvement can provide the key clue to the correct diagnosis. The rash of Lyme disease is a good example.
The bite of the tick fostering Lyme disease may or may not be noticed by its victim, but the initial phase of this bacterial infection is a distinctive skin eruption, ‘erythema chronicum migrans’. Recognition of the eruption can lead to early treatment and prevent later, more serious complications. Due to our changing climate, Lyme disease has already become much more prevalent in regions where it was well established and it is spreading into new and previously unaffected regions of North America.
Another tick-related disease is the newly recognized entity of ‘meat allergy’. This is not an infection; rather, the tick bite leaves behind a substance, an ‘antigen’, to which many people can become ‘sensitized’ – i.e., develop allergy-type antibodies against it. A few may even experience severe allergic reactions following the consumption of beef, lamb or pork, because these meats contain a similar, ‘cross-reacting’ antigen. Skin reactions, such as hives (‘urticaria’) and sudden swelling of the lips and tongue (‘angioedema’), can be the prelude to a more life-threatening, anaphylactic reaction. As the range of the Lone Star tick – the species whose bite in the US has been associated with meat allergy – continues to expand its range across the US due to climate change, serious cases of meat allergy are on the rise.
Mosquitoes transmit a number of serious diseases: malaria, dengue, Zika, West Nile fever and yellow fever to name a few. While the rashes are associated with most of these infections are rather nonspecific, the dermatologist is often called in by other physicians, who are caring for the patients infected by one of these microbes, to help with the diagnosis.
The mosquitoes that carry the West Nile virus are of the Culex genera. These mosquitoes are common to all parts of the US, and represent a familiar experience. Most active in the evening, they emit the annoyingly familiar high pitched hum as they search for available flesh on which to feed. West Nile virus, already widespread in the US, is predicted to continue its dispersion throughout the country as our climates change.
The smaller, Aedes mosquitoes, can carry yellow fever, dengue, zika and chikungunya. These mosquitoes are becoming more prevalent, particularly in the southern parts of the US. While at present few carry these disease-causing viruses, the potential is there for them to be introduced into these mosquito populations. At present, dengue is endemic in Puerto Rico and isolated instances of locally-acquired dengue have been reported in Texas, Florida and even New York. A few cases of locally-acquired Zika infections have also been reported in the two southern states. Both Zika and dengue are predicted to spread northward in coming years.
The Aedes mosquitoes that transmit these infections are much more nefarious than their Culex relations. They are smaller, don’t hum, and bite as often during the daytime as at night. Most critically, they can breed in very small bodies of water – such as that found in the base of a flower pot – making it very difficult to eradicate them from the environment. Vaccines against dengue, Zika and chikungunya are presently under development.
Plague, caused by the bacterium Yersinia pestis, is another dread disease that is predicted to move out of its confined southwestern locale and into the entire western US as this region becomes drier. This projected change of climate will provide a suitable habitat for the rodent species that harbor their plague-bearing fleas. Once again, the skin may provide the key to early diagnosis and treatment. Localized lumps in the skin, which represent infected lymph nodes, (or ‘buboes’), can herald the presence of the infection. Prompt diagnosis and institution of antibiotic therapy can be life-saving.
The drying of the west is also predicted to favor the spread of the fungal infection, coccidiodomycosis, or Valley Fever, from its relatively narrow home in the central valley of California, to the coast and northward. Spores of this fungus lie dormant in the soil until sent aloft by dry winds into the air where they can be inhaled, infecting the lungs (Valley Fever) and, potentially, disseminating to other parts of the body. While many cases are asymptomatic or self-limited infections, some populations, including the immune-suppressed, are at risk for disseminated disease. Prompt diagnosis and antifungal therapy can be life saving.
Sand flies, which harbor the parasite that causes leishmaniasis, are also predicted to spread from Mexico northward into the US. Skin infection is the most common form of leishmaniasis, wherein the bite develops into an ulcerating, non-healing sore. But some species of the parasite produce a potentially fatal form of disseminated infection. Currently, treatments for leishmaniasis are not ideal; they are often of limited efficacy and may be quite toxic.
What To Do?
As a first step, we can prevent reduce our chances of getting bitten by a disease-bearing mosquito or tick, by staying indoors and by keeping our pets free of fleas. When outdoors, we can wear protective clothing and apply repellents to the remaining sites of exposed skin and to our clothing. The CDC recommends using a repellent that contains 20% DEET in tick infested areas, because other repellents which are effective against mosquitoes may not protect against ticks and other insects. Some repellents inactivate sunscreens, hence it is recommended that sunscreens be applied before applying the repellent. There is also evidence that the co-application of DEET repellents with broad spectrum sunscreens containing oxybenzones may result in the increased absorption of both compounds.
To avoid sensitization to tick-bite antigens, the home use of forceps to extract the tick is discouraged, in order to avoid the injection of tick saliva into the wound. Instead, allergists currently recommend that ticks be either removed by medical personal with the proper equipment in the office, or at home by use of rapid freezing methods, such as those butane-containing (and hence flammable) sprays marketed for the home treatment of warts. These sprays can quickly kill the tick and then allowing it to fall off on its own.
Early diagnosis and appropriate treatment can be critical to preventing complications in a number of on these infections.
Stay tuned here for updates on the microbes that are on the move as our planet warms.